Completeness

We have determined the completeness of the SDSS photometric survey
by comparing the number of objects found by the SDSS pipeline to the
number found by the COMBO
survey in a region of sky that has been scanned by both surveys,
and in a second comparison with data from observations of a CNOC survey field.
While the comparison was carried out for DR1 data, the DR2 data are
not significantly different.

Here we show details for the comparison with COMBO data. The region
of sky used here for calibration is COMBO's S11 field, which is
covered by run 1140, camcol 5, fields 151-154 and run 1231 camcol 5
and 6, fields 42-45.

The sky brightness in over half the SDSS is better than it is in
these fields, and the seeing in most of the survey is better than it
is in one of these runs, so the completeness in most of the survey
should be better than what is shown here. See the seeing and sky brightness table for
COMBO/SDSS overlap below.

In the following plots, PSF r-band magnitudes are used for stars,
and Petrosian r-band magnitudes are used for galaxies.

Which objects

Number counts

Fractional completeness

Stars

Galaxies

Note that COMBO's multicolor classification starts to become
incomplete for stars at about one magnitude fainter than
SDSS. Therefore, the SDSS 50% completeness is slightly brighter than
indicated by this plot.

The number of objects above the 5-sigma detection threshold depends
heavily on the seeing and sky brightness in
a given field. The following table summarises the seeing and sky
brightness in the overlap region.

run

camcol

field

seeing ["]

sky brightness [mag/sq. "]

1140

5

151

1.293

20.8210

1140

5

152

1.273

20.8210

1140

5

153

1.280

20.8150

1140

5

154

1.331

20.8060

1231

5

42

1.941

20.7670

1231

5

43

2.051

20.7670

1231

5

44

2.052

20.7700

1231

5

45

2.011

20.7710

1231

6

42

2.010

20.7740

1231

6

43

2.113

20.7750

1231

6

44

2.122

20.7750

1231

5

45

2.033

20.7780

Comparing with the seeing and sky
brightness statistics, most of the SDSS fields have better seeing
than this overlap region. Over half of the fields have lower sky
levels. Therefore, the completeness in most of the survey should be
better than what is shown here